We consider a wireless system consisting of a single-antenna TX, a single-antenna RX, and an \(M\) -elements linear RIS in a two-dimensional Cartesian coordinate system (global reference) as shown in Fig. REF . The RX and TX are located at NF regionNF region, which is usually defined as the range smaller than the Fraunhofer distance \(d_F = 2D^2/\lambda \)  [1]}. Here, \(D\)  and \(\lambda \)  are the dimension of the RIS and the wavelength at the carrier frequency, respectively. of the RIS with the LOS path blocked.This is not a limiting assumption, but only invoked for notational convenience. When the LOS path is present, it can be separated from the RIS path as in [2]}. When the TX and RX are synchronized, the LOS path does not convey any information. We also assume that TX and RX are synchronized (i.e., known clock bias) with known position (\(\mathbf {p}_{\text{tx}}\)  and \(\mathbf {p}_{\text{rx}}\) ) (e.g., RX as calibration agent).
